Generally, two parameters are mentioned when pulse-code modulation (PCM) is used for generating a digital audio data. The first parameter is the sample rate. As the sample rate gets higher, audio quality becomes better. Good audio quality is achieved when the sample rate is 8000 Hz. However, a sample rate of at least 44100 Hz is needed to generate CD-like audio quality. The second parameter is the sample size, which is a number of a unit of bits for representing an audio data. Data stored in the PCM represents amplitude of the audio waveform. When the sample size is 8 bits, the difference between the minimum and maximum values of the amplitude is 255. When the sample size is two bytes of 16 bits, the difference between the minimum and maximum values of the amplitude is 65535, which certainly yields a much better audio quality. However, when more bits are used for representing the data of the audio, the system not only requires more memory, but also needs a better digital signal processor (DSP). Furthermore, when the audio digital data is transmitted via a network, a rather wide bandwidth is occupied and serious overload of the network resource is incurred.
Therefore, a concept of differential pulse-code modulation (DPCM) is introduced. The DPCM records a difference between a current value and a previous value. Compared with the single pulse-code modulation, a data volume generated by using the DPCM method is averagely reduced to 25% of an original data volume. Adaptive differential pulse-code modulation (ADPCM) is a variant of the DPCM. The ADPCM can further reduce the data volume, thereby increasing transmission efficiency. The technology is described in detail in the ITU-T G.726 standard, which shall not be described for brevity. However, in the ITU-T G.726 standard, an audio data having been compressed can only be successfully decompressed and reconstructed by implementing highly sophisticated hardware design to a receiving end applying the ADPCM. Thus, cost of the hardware is rather high to result in a loss of competitiveness. Therefore, one object of the present disclosure is to develop a new technology to solve the foregoing problem effectively.